1. Field of the Invention
This invention relates to a process and circuitry for the electrolytic coloring of anodized articles of aluminum or aluminum alloy in a coloring bath which contains at least one metal salt coloring the aluminum article by using an asymmetrical, substantially sinusoidal a.c. voltage consisting of two separately controllable half-wave trends obtained by corresponding rectification.
2. Discussion of Related Art
Articles of aluminum or aluminum alloys are often used as visible elements in the construction field and in architecture. For example, aluminum is frequently used for facade panels of buildings. In such applications, the panels must be provided with a protective coating and must also lend themselves to coloring.
Thus, German Published Application 19 02 983 describes a process for the production of a colored protective coating on articles of aluminum or aluminum alloys, in which an alternating current is passed through a coloring bath. In this process, the a.c. voltage applied to the coloring bath is made asymmetrical by modulation, i.e. by superimposing a second voltage thereon. This involves considerable technical demand; in particular, a second voltage source is always necessary. In addition, dark colors are only obtained after a relatively long treatment time and by successive treatment first with a symmetrical and then with an asymmetrical a.c. voltage. Modulation also means that the amplitude or frequency of an oscillation changes with time, so that a sinusoidal a.c. voltage of constant frequency is not readily obtained.
Both here and in the following, the term "asymmetrical" is applied to an a.c. voltage or an a.c. current when, although its trend as a function of time is periodic irrespective of shape, the half waves of one direction differ form the half waves of the other direction in their trend and their mean intensity value.
European Patent 0 056 478 describes a process in which the aluminum parts to be colored are treated first with alternating current in one bath containing no coloring metal salts, and then with an a.c. voltage in a second bath containing metal salts which color the aluminum parts. The disadvantage of this process lies in the use of two baths for coloring, in the very high technical demand involved, and in the magnitude of the voltage used (55 to 85 volts). In addition, relatively long coloring times are also required to obtain dark colors.
German Published application 1,930,288 and U.S. Pat. No. 3,669,856 describe a process and circuitry for coloring aluminum alloys by application of an asymmetrical a.c. voltage or a combination of symmetrical and asymmetrical a.c. voltage. In said process, an asymmetrical a.c. voltage applied to the coloring bath, wherein the positive and negative half waves are separately controllable, is produced on the secondary side of a transformer by the division of a current feed path to the coloring bath into two parallel branches each comprising a rectifier directed oppositely to the other and a variable resistor connected in series with the rectifier, or an individually variable voltage controlled rectifier, more especially a thyristor, directed oppositely to the other. These two branches are recombined into a single path prior to the coloring bath. This prior art is attended by the disadvantage that the current path is only divided on the secondary side of the transformer and the current or voltage trend on the secondary side is controlled by resistors or thyristors. However, where a resistance-controlled current source such as this is constructed on an industrial scale, considerable energy outlay is involved in cooling the variable resistors because they heat up enormously. At 10,000 A, the power dissipation occurring amounts to around 50-100 kW. In addition, the use of variable resistors on the secondary side is attended by the disadvantage that, due to the high voltage-dependent load alternation behavior of the coloring bath, the voltage undergoes deformation so that the voltaged trend in the coloring bath is no longer sinusoidal. Since a coloring bath is characterized by an alinear current-voltage curve, this also affects the voltage drop across a series-connected resistance so that a voltage which has a sinusoidal trend before a resistance no longer has that sinusoidal trend after the resistance and hence parallel to the color bath. Neither can a sinusoidal voltage trend be maintained in the coloring bath by control with thyristors.
An object of the present invention is to provide a solution to the aforementioned disadvantages which speeds up the coloring process while guaranteeing uniform coloring and which may be implemented on an industrial scale with simple technical means and which enables an asymmetrical, substantially sinusoidal a.c. voltage with separately controllable positive and negative half waves to be applied to the coloring baths and to be maintained.